The present invention relates generally to fluorescent lamps and more particularly to a fluorscent lamp assembly having an internal magnetic field.
A conventional fluorescent lamp consists of a tubular glass bulb capped by two bases which are fitted with pins to carry electricity to the internal electrodes. Inside the bulb are minute droplets of mercury and an inert gas, usually argon or an argon-neon mixture. The inside of the bulb is coated with phosphor powders which will fluoresce when exposed to ultraviolet radiation.
When a voltage is imposed on the electrodes, electrons will be emitted, ionizing the gas inside the tube. The ionized gas is an electrical conductor and an electron flow in the form of an arc discharge will be established between the electrodes, with the heat of this arc causing the mercury droplets in the bulb to vaporize. The electrons are accelerated by the voltage across the electrodes and will collide with the mercury atoms, exciting them to states of higher energy. As the excited mercury atoms return to their ground state, photons of electromagnetic energy, both in the visible and the ultraviolet ranges, will be emitted. The lamps operate at low pressure to enhance the ultraviolet radiation which exites the phosphor coating to luminance at longer, visible, wavelengths. The resulting light output is not only much higher than that obtained from the visible mercury lines alone, but also results in a continuous spectrum.
One of the problems with fluorescent lamps is the loss of efficiency due to the ultraviolet self-absorption inherent in such lamps. An emitted ultraviolet photon can also collide with a ground state mercury atom and excite it to a higher energy level. As that excited atom returns to its ground state, another ultraviolet photon of the same amount of energy will be emitted. Thus, the ultraviolet photons emitted as a result of electron excitation can be absorbed and re-emitted by mercury atoms as the photons radiate outwardly to the fluorescent coating. Generally, the greater the distance from the point of electron excitation to the fluorescent coating, the greater the number of times that the emitted photon will be re-absorbed and re-emitted. There is no energy loss, of course, in the excitation of a ground state mercury atom by a photon, since the energy absorbed by the atom will be all re-emitted as the excited atom returns to its ground state. However, the density of excited mercury atoms in the tube will be increased by such absorption of photons, thereby increasing the number of collisions of electrons with excited mercury atoms. Such collisions will result in an absorption of energy which will not then be released as ultraviolet radiation.
Various attempts have been made in an effort to reduce the problem of self-absorption by causing the arc discharge to spread out through the bulb, thereby decreasing the average distance from electron-excited mercury atoms to the fluorescent coating. For example: U.S. Pat. No. 4,341,977, discloses a mercury-argon fluorescent lamp with Freon used to cause arc spreading; U.S. Pat. No. 4,311,943 uses glass or quartz fibers, in conjunction with a magnetic-field arc-spreading coil to spread the arc; U.S. Pat. No. 4,311,942 uses an alternating current to create an expanding and contracting magnetic field to spread the electrons in the arc; U.S. Pat. No. 4,177,401 uses a permanent magnet situated so as to cause the arc to rotate about the axis of the lamp; U.S. Pat. No. 4,341,979 shows the use of coils designed to generate a rotating magnetic field in the lamp so that the arc will spread.
In spite of these efforts, the problem still remains of providing an economical and efficient manner of reducing the amount of ultraviolet self-absorption in fluorescent lamps.